Solid state light sources, such as light emitting diodes (LEDs) are increasingly used for a variety of lighting and signalling applications. Advantages of LEDs over traditional light sources, such as incandescent or fluorescent lamps, include long lifetime, high lumen efficiency, low operating voltage and fast modulation of lumen output. For general lighting applications, commonly LEDs are used which emit a Lambertian light-distribution.
However, for many applications, such as automotive front lighting, directional light sources are preferable. Typically, such directionality is achieved through the use of ‘top-emitting’ LEDs; LEDs adapted so as to allow light to escape only in a single direction.
In addition to directionality, highly projective, beam-like, emission is often also desired, requiring an arrangement which can realise high-contrast. For example, ‘low-beam’ automobile front lighting, which projects light only below a certain critical angle, so as to avoid glare for oncoming traffic, requires a sharp horizontal cut-off in emission. A similar requirement is needed for future matrix headlights, with low and high beam functionality. In addition, sharp contrast laterally is also desirable, wherein each LED is able to realise a concrete cut-off not only in height but also in width of the beam, projecting therefore only across a particular segment of the horizon.
High-contrast emission is typically achieved by further collimation of the light emitted from one or more top-emitting LEDs using higher-level optics. In particular, frequently there is used a primary optical element (typically a collimator or reflector) and a secondary optic (typically a lens).
However, in such an arrangement, typically the requirements for the collimation angle and the desired cut off necessitate optical elements having lateral dimensions which significantly exceed those of the LED die beneath, thereby extending the overall footprint incurred by the package. For example, to achieve a collimation angle above and below the normal of 40 degrees, using an input LED die of size 1.0 mm×1.0 mm, a typical collimator, having total internal reflection lens and an air gap between the lens and the light source, would have dimensions of approximately 2.7×1.6×1.6 mm3. For a configuration without an air gap, the dimensions would be approximately 3.5×2.3×2.3 mm3. Similarly, for an open collimator, based upon a reflector rather than upon total internal reflection, the typical dimensions might be 1.5×1.6×1.6 mm3. Thus, it can be seen that the footprint is significantly increased compared to the LED die size.
An arrangement would be desirable therefore which can achieve highly directional light emission, with sharp longitudinal and latitudinal cut-off, but wherein optical elements do not incur a significant expansion to the footprint of the LED package. This would enable greater LED module density within applications, simplified assembly of arrays of modules, as well as increased general flexibility of the packages.